There has been a demand for such an insulating material having excellent adhesive property, which can be used as high-density mounting materials such as semiconductor packages, COL and LOC packages, and MCMs (multi-chip modules), printed wiring materials such as multi-layer FPCs, and aerospace materials.
Generally, epoxy resin or acrylic resin has been mainly used for insulating adhesive material for electric parts/materials, because of its processability under low temperature (below 200.degree. C.) and its easy handling characteristic. These adhesive materials have been also used to adhere a lead frame and an element in the course of manufacturing a semiconductor device.
However, these materials have various problems. For example, while they can be easily handled under low temperature, they have poor heat-resistance under high temperature. In addition, since they have been used in the form of solution, production process involves complexity. Further, adhesion of said adhesive materials requires a long curing process and gas is generated during the curing process. When said adhesive materials are used to adhere a lead frame and a semiconductor device, the generated gas stains the surface of this device, which degrades adhesive effect between the lead frame and the semiconductor device to lower the yield of products.
For these reasons, there has been a strong demand for a high-quality adhesive material to be used as a high-density mounting material, especially used in the case of manufacturing a semiconductor device. Also, there has been a great demand that these adhesive materials have high elastic modulus and low hygroscopic swelling coefficient, when they are used as a base film for a FPC or a TAB carrier tape or resin for composing a laminated sheet.
Concretely, the above-referred TAB carrier tape is internally loaded via sequential steps comprising the following: (1) slitting and punching; (2) laminating of copper foil and curing of adhesive agent; (3) forming of predetermined patterns including steps of resist coating, copper etching, resist stripping; (4) plating; (5) inner-lead bonding; (6) resin sealing; (7) punching; and (8) outer-lead bonding. In order to mount an LSI on a base tape for a TAB carrier tape, it is essential that base film should be of the one having high elastic modulus to sustain the LSI.
Heat is applied in the course of (2) laminating of copper foil and curing of adhesive agent, (5) inner-lead bonding, (6) resin sealing, and (8) outer-lead bonding. Therefore, if a base film of a base tape for a TAB carrier tape has a high coefficient of thermal expansion, it will cause various problems. For example, in the course of (5) inner-lead bonding, connecting position of the base film with the LSI deviates from the predetermined position, because of thermal expansion of the base film. Likewise, in the course of (8) outer-lead bonding, connecting position of the base film with the outer lead deviates from the predetermined position. Further, in the course of (2) laminating of copper foil, the base film is heated together with the copper foil. Therefore, if the base film of the base tape for a TAB carrier tape has different thermal expansion from the copper foil, thermal behavior is also different from each other to cause curling to be generated.
Accordingly, in the course of (5) inner-lead bonding, it is essential that coefficients of thermal expansion of the LSI(silicon wafer) and the base film of the base tape for a TAB carrier tape should be approximate to each other (the LSI has the coefficient of thermal expansion of 0.4.times.10.sup.-5 .degree. C..sup.-1). In the course of (8) outer-lead bonding, it is essential that coefficients of thermal expansion of the outer lead and the base film should be approximate to each other. In the course of (2) laminating of copper foil, it is essential that coefficients of thermal expansion of the copper foil and the base film should be approximate to each other (copper foil has the coefficient of thermal expansion of 1.6.times.10.sup.-5 .degree. C..sup.-1).
Based on the above requirements, it is preferable that a base film of a base tape for a TAB carrier tape should have high elastic modulus and its coefficient of thermal expansion should be about 0.4.about.2.6.times.10.sup.-5 .degree. C..sup.-1. An early materialization of such a base tape for a TAB carrier tape having a base film with the above-referred physical properties has thus been longed for. In the case of a FPC, the base film, which is a long insulating material, is produced via roll-to-roll sequential processes comprising (1) coating with adhesive material and drying, (2) laminating of copper foil and curing adhesive agent, and (3)forming of predetermined patterns comprising steps of resist coating, copper etching, and resist stripping. Generally, since a dimensional change can be seen during an etching step in the course of forming predetermined patterns, it has been necessary to anticipate such a change that may occur in the subsequent processes, when circuits are designed.
The above-referred dimensional change is generated by; (a) a dimensional change caused by absorbing and releasing moisture into and out of the base film used as an insulating material of the FPC, (b) distortion caused by the difference of thermal expansion between copper foil and the base film generated in the course of laminating copper foil, and (c) distortion caused by tension generated in the course of executing predetermined processes.
In the case that a coefficient of thermal expansion of the base film is constant, a dimensional change caused by the factor (b) can be easily anticipated when circuits are designed. The Dimensional change caused by the factor (c) can also be easily anticipated when circuits are designed, since it can easily be controlled by controlling tension in each of the sequential processes.
However, since a base film is washed and dried repeatedly in the FPC manufacturing process, it has been quite difficult to properly control the dimensional change caused by absorbing and releasing moisture into and out of the base film cited in the above factor (a). In addition, since conventional base films used as insulating materials had high coefficient of hygroscopic swelling, dimension of a base film was considerably changed by absorbing and releasing moisture in the FPC manufacturing processes, which caused a problem.
Polyimide having an excellent heat-resistant property among a wide variety of organic polymers has been extensively used in various fields ranging from the aerospace field to the electronic commnication fields. However, there has been a great demand for such kind of polyimide having not only an excellent heat-resistant property, but also various physical properties to solve above properties.
In order to achieve such polyimide which can solve the above problems, it is essential that low thermal expansive property should be materialized by way of forming a mainchain of polyimide as stiff as possible. Polyimide synthesized by the use of pyromellitic acid, which has the stiffest structure among all the existing acids, may easily achieve high elastic modulus. However, because of high polarization of imino group, polyimide can not gain a property of low water absorption. In order to lower water absorption, it might be possible to introduce fluorine into polyimide, however, the process may increase the production costs. Consequently, there has been actually no polyimide, which can contain satisfying physical properties including high elastic and low hygroscopic swelling coefficient.